The invention concerns a transfer medium, in particular one made of paper, which has been printed on using sublimable dispersion colors by the halftone process, and which is suitable for sublimation heat transfer printing on anodized aluminum. The invention also concerns a process for carrying out the heat transfer printing.
Aluminum and aluminum based alloys in the form of finished or semi-finished products are anodized to provide them with superior corrosion and wear resistance and to give them a decorative appearance.
The article or semi-finished product is immersed in an electrolyte, generally comprising dilute sulphuric acid, occasionally with oxalic acid added. Less often, the electrolyte comprises only dilute oxalic acid dilute phosphoric or dilute chromic acid.
An oxide layer thus builds up on the surface of the item, which becomes the anode, under the influence of an electrical current. The current mainly takes the form of direct current (DC), less often using alternating current (AC) or by alternating between or overlapping AC and DC currents.
These oxide layers comprise in general a very thin, almost pore-free dielectric base layer, i.e., the so-called barrier layer, and a finely porous layer on top of the barrier layer. The barrier layer is self-regenerative, converting aluminum to aluminum oxide at the same rate as the top layer is produced by anodic oxidation.
The top layer is made up of bundles of fibers, oriented essentially perpendicular to the surface of the metal. The fibers are approximately 350 A in diameter and have a pore in the center approximately 150 A in diameter which reaches down to the barrier layer.
The finely porous top layer, formed using dilute sulphuric acid as the electrolyte and using direct current, is generally transparent and colorless.
There are many processes which can be employed to color this top oxide layer on anodized aluminum. These processes fall into four groups, depending on the way they produced the color effect.
1. By means of special electrolytes, e.g., aqueous solutions of carbonic acids or sulphonic acids. PA1 2. By depositing metals in the pores in the bundles of fibers which make up the top layer of a transparent, colorless oxide layer by the application of alternating current in an aqueous solution of metallic salts. PA1 3. By depositing inorganic pigments or organic coloring agents in the pores in the bundles of fibers which make up the top layer of a transparent, colorless anodic oxide layer by immersion in warm solutions containing the coloring agents. PA1 4. By depositing organic coloring substances in the pores in the bundles of fibers which make up the top layer of a transparent, colorless anodic oxide layer such that sublimable, hydrolysis resistant colorants are printed on a transfer medium such as paper. The colorants are then drawn into the pores in the bundle of fibers in the absorbant top oxide layer on coming into contact with that layer and with the assistance of heat. PA1 (a) On printing the transfer medium, such as the paper, by means of conventional printing methods, in particular by the offset printing method, coloring agents with different specific weights are transferred non-uniformly. As far as transfer printing using the trichromatic principle is concerned, such a result is a great disadvantage. The sublimable dispersion colorants normally employed have different chemical structures and can have very different specific weights. PA1 (b) The amount of colorant sublimated as a function of time at constant temperature can, in the case of the conventional sublimable colorants, vary considerably. This variation causes the fine pores in the oxide layer to be filled mainly with the coloring agent which is transferred fastest or to the greatest extent at the given temperature, i.e., sublimized from the transfer medium and taken up by the pores. PA1 (c) On sealing the pores by treatment in hot deionized water or steam after the color transfer stage, a portion of colorants at the surface of the oxide layer is washed away before the pores are closed by the formation of aluminum hydroxides. PA1 (a) Dispersion colorants based on anthrachinon, where either H.sup.-, OH.sup.-, or amino or amido groups with at least one active hydrogen atom, occupy at least one position 1, 4, 5 or 8; PA1 (b) Azo colorants with an OH.sup.- group in the ortho position; or PA1 (c) Colorants with a 1, 3 indandion group.
After the color transfer has occurred by depositing the colorant in the pores in the bundle of fibers which make up the oxide layer, this porous layer is closed or sealed by treatment in hot, deionized water or steam. As a result of this sealing process, at least a part of the Al.sub.2 O.sub.3 in the newly formed oxide layer transforms to AlOOH, or pseudo-boehmite.
On examining the four different processes for coloring anodized aluminum, it becomes clear that multi-colored, patterned or image bearing oxide layers can be produced particularly advantageously at a commercially acceptable cost by using the fourth of the above processes. In this process, sublimable colorants can be transferred from a transfer medium, made, for example, of paper, plastic or metal foil, with the aid of heat and contact pressure, by so called heat transfer pressure.
This process has been known for some time now (see, for example, the German Patent DT-OS No. 15 21 849), but has not been able to develop into a process with widespread application. The reason for this is that the said process suffers from the serious disadvantage that the colorant or mixture of colorants on the transfer medium, such as paper, do not allow images which are true to color to be produced on anodized aluminum by means of conventional processes such as photogravure, litho, relief and screen printing.
The following factors caused the changes in color with respect to the original:
When equal amounts of the various colorants have been sublimated or transferred, this effect, which removes excess colorant, is of no importance. When different amounts of colorants are transferred, due to different rates of sublimation at constant temperature, the washing off of colorant during sealing removes a large amount of the colorant which was transferred last and therefore lies next to the surface of the oxide. Color mixtures employed for coloring on the trichromatic principle then produce an undesirable shift in color tone, and the resultant image differs in color from the original.
The German patent mentioned above also reveals that this process can be carried out using coloring agents from the range of conventional sublimable dispersion colorants used in the textile industry. These agents include:
The processes which represent the present state of the art of transferring images to non-anodized aluminum permit the production of patterns made up of areas of the same color where, for example, continuous changes in color tone or so called halftones are not possible. Only in special cases, therefore, can this process produce actual "pictures" on anodized aluminum.
The main disadvantage of the known processes can be seen to be that the printer who produces the transfer medium, for example, by offset, relief or photogravure printing or by some other suitable printing process, is forced to check the coloring on the product he makes, i.e., the transfer medium. In other words, the printer determines if the pattern corresponds in color with that on the printed transfer medium.
The present state of the art overcomes this disadvantage by using a suitable colorant or mixture of colorants for each color. However, on using known processes, even this improvement limits the number of colors which anodized aluminum can take to the range of sublimable colorants.